You can charge as many car batteries as needed, all at the same time, as long as the connections between them are joined together to create a parallel circuit. That means all the positive connections are linked together and all the negative connections are linked together. If the connections between batteries are installed positive to negative, and that pattern repeats, this creates a series circuit and the voltage is increased by the number of batteries connected. For example, three 12-volt batteries connected in series will produce 36 volts and is unsafe to charge with a 12-volt battery charger.
Use a wrench remove the battery from the vehicles that need charging. Mix a small amount of baking soda with water and use it to clean the battery. The baking soda will react and neutralize the surface acid that accumulates on the battery cases. Rinse the cases with clean water.
Connect two or more batteries with jumper cables, positive to positive, and negative to negative. Install the battery charger connections in the same manner and set the charger to slow charge the batteries. Leave the charger on overnight, if possible. If any of the batteries becomes overheated and spews acid during the charging process, the battery is internally shorted. Replace the battery.
Clean the battery cable connections that are on the car with baking soda, and reinstall the battery. Tighten the clamps with a wrench and start the car. If the battery fails to start the car, it is defective and unable to be charged. Replace the battery.
